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Lecture #13

ANNOUNCEMENTS Quiz #2 next Friday (2/23) will cover the following: carrier action (drift, diffusion, R-G) continuity & minority-carrier diffusion eq’ns MS contacts (electrostatics, I-V characteristics) Review session will be held Friday 2/16 at 12:30PM

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Lecture #13

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  1. ANNOUNCEMENTS Quiz #2 next Friday (2/23) will cover the following: carrier action (drift, diffusion, R-G) continuity & minority-carrier diffusion eq’ns MS contacts (electrostatics, I-V characteristics) Review session will be held Friday 2/16 at 12:30PM No office hour or coffee hour today  Lecture #13 • OUTLINE • Metal-semiconductor contacts (cont.) • practical ohmic contacts • small-signal capacitance • Introduction to pn junction diodes • Reading: Finish Ch. 14, Start Ch. 5 EE130 Lecture 13, Slide 1

  2. Practical Ohmic Contact • In practice, most M-S contacts are rectifying • To achieve a contact which conducts easily in both directions, we dope the semiconductor very heavily  W is so narrow that carriers can tunnel directly through the barrier EE130 Lecture 13, Slide 2

  3. Band Diagram for VA0 Equilibrium Band Diagram q(Vbi-VA) qVbiFBn EFM EFM Ec, EFS Ec, EFS Ev Ev EE130 Lecture 13, Slide 3

  4. Specific Contact Resistivity, rc • Unit: W-cm2 • rc is the resistance of a 1 cm2 contact • For a practical ohmic contact,  want small FB, large ND for small contact resistance EE130 Lecture 13, Slide 4

  5. N = dopant concentration in surface layer a = width of heavily doped surface layer • fM engineering • Impurity segregation via silicidation • Dual ( low-fM / high-fM ) silicide technology A. Kinoshita et al. (Toshiba), 2004 Symp.VLSI Technology Digest, p. 168 Approaches to Lowering fB • Image-force barrier lowering Df qfBo EF EC n+ Si metal  Very high active dopant concentration desired • Band-gap reduction • strain • germanium incorporation A. Yagishita et al. (UC-Berkeley), 2003 SSDM Extended Abstracts, p. 708 M. C. Ozturk et al. (NCSU), 2002 IEDM Technical Digest, p. 375 EE130 Lecture 13, Slide 5

  6. Voltage Drop across an Ohmic Contact • Ideally, Rcontact is very small, so little voltage is dropped across the ohmic contact, i.e.VA0V • equilibrium conditions prevail EE130 Lecture 13, Slide 6

  7. Review: MS-Contact Charge Distribution • In a Schottky contact, charge is stored on either side of the MS junction • The applied bias VA modulates this charge EE130 Lecture 13, Slide 7

  8. Schottky Diode: Small-Signal Capacitance • If an a.c. voltage va is applied in series with the d.c. bias VA, the charge stored in the Schottky contact will be modulated at the frequency of the a.c. voltage • displacement current will flow: EE130 Lecture 13, Slide 8

  9. Using C-V Data to Determine FB Once Vbi and ND are known, FBn can be determined: EE130 Lecture 13, Slide 9

  10. Summary Ec Ec EF EF Ev Ev Ec Ec EF EF Ev Ev Since it is difficult to achieve small FB, practical ohmic contacts are achieved with heavy doping: Ec EF Ec EF Ev Ev Charge storage in an MS junction  small-signal capacitance: EE130 Lecture 13, Slide 10

  11. pn Junctions – + I V I N P V Reverse bias Forward bias diode symbol EE130 Lecture 13, Slide 11

  12. Terminology Doping Profile: EE130 Lecture 13, Slide 12

  13. Idealized Junctions EE130 Lecture 13, Slide 13

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